Archive for the ‘Oakland hazards’ Category

Earlier this year I took a peek at the mitigation work going on at the former pyrite mine at the head of McDonell Street, in the Leona Heights neighborhood. The workers had laid down lovely geofabric, seeded grass and paved the creek bed with boulders. They were also patching infant gullies. I predicted it wouldn’t last. Sure enough, on Saturday large expanses of black plastic were shielding parts of the locality.

Farther down, the patchwork had massively failed.

This was after a normal December rainfall. We’re looking at El Niño rainfall during the rest of the winter that will be substantially above average.

On the good side, I can report that the water downstream at Twitter Court wasn’t an ugly orange. Whether that’s due to larger runoff volume or success in muffling the acid drainage, I’m not qualified to say. Think positively as you walk past the mine site on your way up the fire road into Leona Heights Park.

On Grizzly Peak Boulevard, pretty much right above the Caldecott Tunnel, there’s a little old fire road that heads downhill to the west. I poked my nose down it the other day. The whole area has excellent exposures of the Claremont chert, starting with the roadside.

It’s real nice right now. The ground is moist and makes for quiet walking. Pine needles smell great. The rock is pretty.

There’s a spot where a lot of loose rock has tumbled down. The Claremont can be crumbly, because it’s so brittle, even though the stone itself is rather hard. The loose stuff is good for collecting a specimen if you’re into that. Unlike the bleached stone exposed along the ridgetop, there’s some variety here, including the black, kerogen-rich stuff that has made this formation, like its larger cousin the Monterey Shale, good petroleum source rock.

During the Caldecott Tunnel dig, this formation leaked significant amounts of oil and gas into the working space. Precautions had to be taken. The same black Claremont crops out at Alum Rock, as I showed you a few years ago, as well as at the Calaveras Dam site.

That’s all fun. But the road’s cut off by a washout ripped into the hillside, a twisted galvanized drainpipe sprawled along its path. At some risk, I scrambled across it and noted that at its floor lies the Claremont chert, which has its bedding planes oriented only slightly steeper than the gully. Treacherous ground. I don’t recommend that you follow me.

And just beyond it is another gully, somewhat bigger but not eroding as actively. Giving up on the fire road, I scrambled up to Grizzly Peak Boulevard and this is what’s at the top of that gully.

At the top of the active washout is this innocuous-looking street drain.

As far as I can tell, every one of these cute drains is carving gouges into the hillside. This one points toward the Parkwood condos.

Can’t we do better than this?

Perhaps our children can revise the old playground song to “Oakland hills are falling down.”

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)

Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.

These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.

This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.

Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.

Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.